Ratios In Lakes Research Articles

Influence of social and environmental drivers on nutrient concentrations and ratios in lakes: A comparison between China and Europe

Global anthropogenic flows of nitrogen (N) and phosphorus (P) have exceeded planetary boundaries with significant implications for pollution of the freshwater resources in the world. Understanding the global patterns and drivers of N and P concentrations and their ratios in the lakes could help design more effective management and remediation strategies to mitigate the impacts of eutrophication. While a suite of drivers are associated with the sources of nutrients, their transport and internal processes that control concentrations of N and P in the lakes, much less is known about the relative importance of different drivers in explaining spatial variations of lake nutrients and ratios. In this study, we compared N and P concentrations and their ratios in the lakes across China and Europe and examined the differences of dominant environmental and social drivers on lake nutrients. Our comparison showed that total nitrogen (TN) and total phosphorus (TP) concentrations were much higher in the lakes in China compared to those in Europe (i.e., TN: 1.13 mg/L in China vs. 0.64 mg/L in Europe; TP: 35.83 μg/L in China vs. 19.38 μg/L in Europe, the median value). However, lake N/P ratios for both regions were not statistically different. Concentrations of TN and TP showed decoupling in both regions, with the majority of lakes having high N/P mass ratios when evaluated by the commonly accepted threshold of 23 (i.e., 61% in China and 68% in Europe), indicating that phytoplankton are more P limited relative to N. Agricultural activity in the lake catchment is an important predictor for both nutrient concentrations and their ratio in Europe. This reflects successful investments in infrastructure and policy prescriptions in addressing point sources of pollution. In comparison, lake depth and water residence time are important in the decoupling of N and P concentrations in China. The regional difference between the dominant drivers can provide important insights into development of effective water pollution control measures. It is necessary for policy makers and water resource managers to be aware of large-scale imbalance of nutrients in lake due to the potential environmental consequences. A set of spatially flexible policies for water quality controls would be beneficial for sustaining the ecological integrity and future health of lakes.

Influence of N:P Ratio of Water on Ecological Stoichiometry of Vallisneria natans and Hydrilla verticillata

Eutrophication is one of the major threats to shallow lake ecosystems, because it causes large-scale degradation of submerged plants. N:P ratio is an important indicator to estimate nutrient supply to water bodies and guide the restoration of submerged plants. The massive input of N and P changes the structure of aquatic communities and ecological processes. However, the mechanism underlying the influence of changes in N and P content and the N:P ratio of a water body on the growth of submerged plants is still unclear. In this study, we simulated gradients of water N:P ratio in lakes in the middle-lower reaches of the Yangtze River using outdoor mesocosm experiments. Using established generalized linear models (GLM), the effects of total nitrogen (TN) content and N:P ratio of water, phytoplankton and periphytic algae biomass, and relative growth rate (RGR) of plants on the stoichiometric characteristics of two widely distributed submerged plants, Hydrilla verticillata and Vallisneria natans, were explored. The results reveal that changes in water nutrient content affected the C:N:P stoichiometry of submerged plants. In a middle-eutrophic state, the stoichiometric characteristics of C, N, and P in the submerged plants were not influenced by phytoplankton and periphytic algae. The P content of H. verticillata and V. natans was positively correlated with their relative growth rate (RGR). As TN and N:P ratio of water increased, their N content increased and C:N decreased. These results indicate that excessive N absorption by submerged plants and the consequent internal physiological injury and growth inhibition may be the important reasons for the degradation of submerged vegetation in the process of lake eutrophication.

Shoring up the foundations of production to respiration ratios in lakes

AbstractThe ratio of primary production to ecosystem respiration rates (P:R ratio) is an ostensibly simple calculation that is used to characterize lake function, including trophic status, the incorporation of terrestrial organic carbon into lacustrian food webs, and the direction of carbon dioxide (CO2) flux between a lake and the atmosphere. However, many predictive links between P:R ratios and lake ecosystem function stem from a historically plankton‐centric perspective and the common use of the diel oxygen curve approach. We review the evolution of the use of P:R ratios and examine common assumptions underlying their application to (1) eutrophication, (2) carbon flux through lake food webs, and (3) the role of lakes in the global carbon budget. Foundational P:R studies have been complicated principally by the following: most P:R ratios were calculated from mid‐lake measurements and failed to incorporate nonplanktonic dynamics; there has been confusion regarding the food web implications when P:R ≠ 1; and CO2 fluxes between lakes and the atmosphere are influenced by nonmetabolic processes. We argue for a re‐assessment, or shoring up, of several fundamental assumptions that continue to guide metabolism research in lakes by accounting for mixing, benthic‐littoral processes, groundwater fluxes, and abiotic controls on gas dynamics to better understand lake food webs and accurately integrate lake ecosystems into landscape‐scale carbon cycling models.

Spatial-temporal Variations and Driving of Nitrogen and Phosphorus Ratios in Lakes in the Middle and Lower Reaches of Yangtze River

In spring and summer of 2018, 26 lakes in the middle and lower reaches of the Yangtze River were studied to determine the temporal and spatial characteristics of nitrogen and phosphorus ratios (TN/TP) and their influencing factors. The differences in nitrogen and phosphorus ratios in different types of lakes (including water-psaaing lakes, deep reservoirs and eutrophic lakes) and in different seasons were analyzed in terms of the sources of the lakes, lake depth, suspended particulate matter concentrations, and phytoplankton levels. The average TN/TP was 21.52±14.28 in spring and 21.73±23.78 in summer. The TN/TP varied significantly in different types of lakes. The TN/TP ratios in water-passing lakes, deep reservoirs and eutrophic lakes were 20.41±9.25, 40.97±33.37, and 14.38±7.40 during spring, and were 22.62±6.48, 96.38±45.91, and 10.91±4.44 during summer, respectively. The TN/TP of the water-passing lakes and deep reservoirs increased significantly in summer, while that of the eutrophic lakes decreased significantly, which indicates that TN/TP changes and lake nutritional status are closely related. The source of nutrients in lakes and reservoirs affects the TN/TP. The TN/TP of lakes and reservoirs had a significant correlation with the lake depth in both spring and summer, indicating that lake depth is a key factor affecting the ratio of nitrogen and phosphorus. In addition, in eutrophic lakes with higher absolute nutrient concentrations, TN/TP has less effect on phytoplankton, while in deep-water lakes with lower absolute nutrient concentrations, TN/TP can determine the growth of phytoplankton limited by phosphorus. Therefore, the governance strategy of lakes in the middle and lower reaches of the Yangtze River should prioritize phosphorus control. Local digging, controlling non-point source pollution, sediment dredging, and changing fishery production methods can be applied to improve the ecological quality of the eutrophic lakes.

Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes

Lanthanum-modified bentonite (LMB) is a widely used phosphorus-inactivating agent in lakes. However, dissolved organic carbon (DOC) exists ubiquitously in lakes, and its influence on phosphate binding is still not adequately understood. Our results showed that both phosphate and DOC can be adsorbed by LMB. The Langmuir adsorption maxima of phosphate and DOC were 9.06 mg P/g and 5.31 mg C/g, respectively, generating a C/P molar ratio ∼1.5. When phosphate and DOC coexisted at this ratio, the adsorption of phosphate was not influenced by DOC and vice versa. However, the phosphate capture by LMB was significantly reduced by raising the ratio above ∼9, and the reduction was increased with increasing the ratio. Once adsorbed by LMB, phosphate was essentially not desorbed by DOC, while adsorbed DOC can be mostly liberated by phosphate. It is deemed that phosphate can interact preferentially with La on LMB. However, DOC can still be adsorbed by LMB, even after LMB was saturated with phosphate, which was attributed to (i) the high coordination capacity of La; (ii) the interaction of DOC with the hydroxyl group(s) of the adsorbed phosphate via hydrogen bonding; and (iii) the interaction of DOC with the La sites unoccupied by phosphate. We proposed that LMB can be applied in the season (time) when the DOC/P ratio in lakes is low enough to facilitate the adsorption of phosphate, which will no longer be released into water, even after the C/P ratio is raised later.

Impact of human activities and vegetation changes on the tetraether sources in Lake St Front (Massif Central, France)

The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) has been shown to correlate with mean annual air temperature and is increasingly used for paleoclimate reconstructions, in particular in lakes. Numerous studies have reported in situ production of brGDGTs in lakes. These brGDGTs have different distributions compared with those produced in soils and their mixing hampers paleoclimate reconstructions. Very few tools exist to determine brGDGT sources in the present and to trace their changes in the past linked with environmental changes of climatic or anthropogenic origin. While human activities are known to affect both soil and aquatic ecosystems, particularly bacterial communities, the specific impacts on brGDGT distributions are poorly investigated. High resolution analyses of brGDGTs were carried out on Holocene sediments and catchment soils of Lake St Front (Massif Central, France) in association with sedimentological, palynological, and geochemical analyses. Comparison of brGDGT distributions in sediments and soils revealed their mixed origin. For the first time, we tested the reliability of the ΣIIIa/ΣIIa ratio in lakes which indicated a gradual shift from aquatic to terrigenous brGDGT sources over the Holocene. This shift was supported by sedimentological and geochemical indices. Three events with a high proportion of terrigenous brGDGTs (6–5.5, 2.8–2.5, and 2–0.2 kyr cal BP) coincide with changes in vegetation in the catchment area, driven by climate and/or human activities. This suggests that vegetation modifications in the watershed impact brGDGT distributions and may thus bias brGDGT-based paleoclimatic reconstructions.

A landscape perspective of Holocene organic carbon cycling in coastal SW Greenland lake-catchments

Arctic organic carbon (OC) stores are substantial and have accumulated over millennia as a function of changes in climate and terrestrial vegetation. Arctic lakes are also important components of the regional C-cycle as they are sites of OC production and CO2 emissions but also store large amounts of OC in their sediments. This sediment OC pool is a mixture derived from terrestrial and aquatic sources, and sediment cores can therefore provide a long-term record of the changing interactions between lakes and their catchments in terms of nutrient and C transfer. Sediment carbon isotope composition (δ13C), C/N ratio and organic C accumulation rates (C AR) of 14C-dated cores covering the last ∼10,000 years from six lakes close to Sisimiut (SW Greenland) are used to determine the extent to which OC dynamics reflect climate relative to lake or catchment characteristics. Sediment δ13C ranges from −19 to −32‰ across all lakes, while C/N ratios are <8 to >20 (mean = 12), values that indicate a high proportion of the organic matter is from autochthonous production but with a variable terrestrial component. Temporal trends in δ13C are variable among lakes, with neighbouring lakes showing contrasting profiles, indicative of site-specific OC processing. The response of an individual lake reflects its morphometry (which influences benthic primary production), the catchment:lake ratio, and catchment relief, lakes with steeper catchments sequester more carbon. The multi-site, landscape approach used here highlights the complex response of individual lakes to climate and catchment disturbance, but broad generalisations are possible. Regional Neoglacial cooling (from ∼5000 cal yr BP) influenced the lateral transfer of terrestrial OC to lakes, with three lakes showing clear increases in OC accumulation rate. The lakes likely switched from being autotrophic (i.e. net ecosystem production > ecosystem respiration) in the early Holocene to being heterotrophic after 5000 cal yr BP as terrestrial OC transfer increased.

An assessment of the Ca weathering sources to surface waters on the Precambrian Shield in central Ontario

There is increasing concern over the negative ecological impacts caused by falling calcium (Ca) concentrations in lakes, particularly in central Ontario, Canada. Forecasting regional changes in lake Ca concentrations relies on accurate estimates of mineral weathering rates that are not widely available. In this study, bulk atmospheric deposition, surface water and soil chemistry along with 87Sr/86Sr isotope measurements were used to provide regional insight into weathering controls on Ca concentrations in lakes. Regionally, Ca concentrations in 90% of 129 lakes sampled in central Ontario were <0.1 mmol L−1 and the Ca/Sr ratio in lakes increased and the K/Sr ratio decreased with increasing Sr concentration, which is indicative of greater Ca sources from calcite or apatite in the higher Ca lakes. Significant relationships between 87Sr/86Sr ratios and Ca/Sr rations in dilute acid (0.1 M HCl) soil extracts are also indicative of the presence of trace amounts of calcite or apatite in surficial soils. Within the low (<0.7 mmol L−1) Ca lakes, defined in this study that are considered most at risk from falling Ca concentrations, 87Sr/86Sr ratios fell within the range observed in weak acid soil extracts and were also significantly related to Ca/Na and K/Sr ratios in surface waters. There were large inconsistencies however, between Ca/Na ratios and Ca/Sr in surface waters and soil acid extracts that suggest differences in 87Sr/86Sr ratios in surface waters of the low Ca lakes do not simply reflect differences in Ca derived from non-silicate minerals in surficial soils and that that Ca sources from deeper soil or bedrock are also important contributors to surface water Ca in these low Ca lakes.

Patterns of nutrient dynamics in Adirondack lakes recovering from acid deposition.

With decreases in acid deposition, nitrogen : phosphorus (N:P) ratios in lakes are anticipated to decline, decreasing P limitation of phytoplankton and potentially changing current food web dynamics. This effect could be particularly pronounced in the Adirondack Mountains of New York State, a historic hotspot for effects of acid deposition. In this study, we evaluate spatial patterns of nutrient dynamics in Adirondack lakes and use these to infer potential future temporal trends. We calculated Mann-Kendall tau correlations among total phosphorus (TP), chlorophyll a, dissolved organic carbon (DOC), acid neutralizing capacity (ANC), and nitrate (NO3- ) concentrations in 52 Adirondack Long Term Monitoring (ALTM) program lakes using samples collected monthly during 2008-2012. We evaluated the hypothesis that decreased atmospheric N and S deposition will decrease P limitation in freshwater ecosystems historically impacted by acidification. We also compared these patterns among lake watershed characteristics (i.e., seepage or lacking a surface outlet, chain drainage, headwater drainage, thin glacial till, medium glacial till). We found that correlations (P<0.05) were highly dependent upon the different hydrologic flowpaths of seepage vs. drainage lakes. Differentiations among watershed till depth were also important in determining correlations due to water interaction with surficial geology. Additionally, we found low NO3- :TP (N:P mass) values in seepage lakes (2.0 in winter, 1.9 in summer) compared to chain drainage lakes (169.4 in winter, 49.5 in summer) and headwater drainage lakes (97.0 in winter, 10.9 in summer), implying a high likelihood of future shifts in limitation patterns for seepage lakes. With increasing DOC and decreasing NO3- concentrations coinciding with decreases in acid deposition, there is reason to expect changes in nutrient dynamics in Adirondack lakes. Seepage lakes may become N-limited, while drainage lakes may become less P-limited, both resulting in increased productivity. Long-term measurements of TP and chlorophyll a from the Adirondacks are needed to inform how future decreases in atmospheric N and S deposition will influence the trophic status of lake ecosystems throughout the region.

Numerical Exploration of the Planktonic to Benthic Primary Production Ratios in Lakes of the Baltic Sea Catchment

Autotrophic structure refers to the partitioning of whole-ecosystem primary production between benthic and planktonic primary producers. Autotrophic structure remains poorly understood especially because of the paucity of estimates regarding benthic primary production. We used a conceptual model for numerically exploring the autotrophic structure of 13 hemiboreal lakes situated in the Baltic Sea catchment. We also used diel variations in primary production profiles to graphically evaluate levels of light and/or nutrient limitation in lakes. The input morphometric data, light extinction coefficients and dissolved carbon parameters were mostly obtained from in situ measurements. Results revealed that cross- and within-lake autotrophic structure varied greatly: one lake was clearly dominated by benthic production, and three lakes by phytoplankton production. In the rest, phytoplankton production was generally dominant but switch to benthic dominance was possible. The modelled primary production profiles varied according to lake water clarity and bathymetry. Our results clearly indicate that the relative contribution of benthic primary production to whole-lake primary production should be taken into account in studies about hemiboreal and boreal lakes.

N‐limited consumer growth and low nutrient regeneration N:P ratios in lakes with low N deposition

Nutrient limitation of primary producers and their consumers can have a large influence on ecosystem productivity. The nature and strength of nutrient limitation is driven both by external factors (e.g., nutrient loading) and internal processes (e.g., consumer‐driven nutrient regeneration). Here we present results from a field study in 10 low productive headwater lakes in northern subarctic Sweden, where nitrogen (N) deposition is low and phytoplankton is primarily N‐limited. We assessed the carbon:nitrogen:phosphorus (C:N:P) stoichiometry of seston and zooplankton and estimated the N:P ratio of consumer‐driven nutrient regeneration. Based on stoichiometric models, the estimated elemental imbalances between seston and zooplankton suggest that zooplankton were mainly N‐limited and regenerated nutrients with low N:P ratios (median 11.9, atomic ratio). The predicted N:P regeneration ratios were consistent with results from phytoplankton nutrient limitation bioassays in mid‐summer, i.e., the N:P regeneration was predicted to be low when phytoplankton were N‐limited, and high when phytoplankton were P‐limited. During other seasons, when water discharge was high, nutrient loading from the surrounding catchments apparently had the strongest effect on phytoplankton nutrient limitation. We propose that lakes with higher N:P ratios than the open ocean is an effect of N deposition, that N‐limitation of consumers and phytoplankton is further enhanced by low nutrient regeneration N:P ratios, and that in the absence of N deposition, lake and ocean N:P stoichiometry are similar.

N2 fixation exceeds internal nitrogen loading as a phytoplankton nutrient source in perpetually nitrogen-limited reservoirs

AbstractBiological N2 fixation hypothetically could balance the N∶P supply ratio in lakes and reservoirs, causing P to be the ultimate limiting element. However, the efficiency and time scale of this mechanism has been poorly studied and remains largely unknown. We compared the relative rates of N supply to the epilimnion of 3 reservoirs of similar age and morphology but with differing external nutrient inputs. Our objective was to determine if N2-fixation rates in these reservoirs exceeded other internal N supply pathways during the growing season, a situation that would suggest that these systems are still evolving in terms of nutrient-limitation status. Phytoplankton N2-fixation rates ranged from 3.4 to 16 g N m−2 y−1 in the reservoirs. These rates are among the highest N2-fixation rates reported in the scientific literature. Measureable N2 fixation also occurred in sediments in contact with reservoir epilimnia (0.85–1.5 g N m−2 y−1). Sediment N2-fixation rates exceeded rates of N regeneration from epi...

Nutrient ratios, differential retention, and the effect on nutrient limitation in a deep oligotrophic lake

Nutrient ratios have been related to nutrient limitation of algal growth in lakes. Retention of nutrients in lakes, by sedimentation and by denitrification, reduces the nutrient concentrations in the water column, thereby enhancing nutrient limitation. Differential retention of nitrogen and phosphorus alters their ratios in lakes and thereby contributes to determine whether nitrogen or phosphorus limits algal growth. We examined the relationships between differential nutrient retention, nutrient ratios, and nutrient limitation in Lake Brunner, a deep oligotrophic lake. The observed retention of nitrogen (20%) and phosphorus (47%) agreed with predictions by empirical equations from literature. As a result of differential retention with a much larger proportion of phosphorus retained than that of nitrogen, the nitrogen:phosphorus ratio was higher in the lake (69) than in the inflows (46). While the mean ratio in the inflows suggested no or only moderate phosphorus limitation, the lake appeared to be severely phosphorus limited. Combining empirical equations from literature that predict nitrogen and phosphorus retention suggests that the nitrogen:phosphorus ratio is enhanced by greater retention of phosphorus compared to nitrogen only in deep lakes with relatively short residence times, such as Lake Brunner. In contrast, in most lakes differential retention is expected to result in lower nitrogen:phosphorus ratios.

Lake Champlain 2010: A summary of recent research and monitoring initiatives

Lake Champlain shares a geological history with the Great Lakes and, as part of the St. Lawrence drainage, also shares biological and ecological similarities. The complex bathymetry and extensive shoreline provide a variety of lacustrine habitats, from deep oligotrophic areas to shallow bays that are highly eutrophic. The large basin:lake ratio (19:1) makes Lake Champlain vulnerable to impacts associated with land use, and in some parts of the lake these impacts are further exacerbated by limited water exchange among lake segments due to both natural and anthropogenic barriers. Research in Lake Champlain and the surrounding basin has expanded considerably since the 1970s, with a particularly dramatic increase since the early 1990s. This special issue of the Journal of Great Lakes Research brings together 16 reports from recent research and monitoring efforts in Lake Champlain. The papers cover a variety of topics but primarily focus on lake hydrodynamics; historical and recent chemical changes in the lake; phosphorus loading; recent changes in populations of phytoplankton, zooplankton, and fishes; impacts of invasive species; recreational use; and the challenges of management decision-making in a lake that falls within the legal jurisdictions of two U.S. states, one Canadian province, two national governments, and the International Joint Commission. The papers provide not only evaluations of progress on some critical management issues but also valuable reference points for future research.

Evaporative enrichment of oxygen-18 and deuterium in lake waters on the Tibetan Plateau

Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales. Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau. However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes, contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset, the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic record of downcore carbonate sediments in the region.

Mathematical modeling of the aquatic macrophyte inputs of mid-chain n-alkyl lipids to lake sediments: Implications for interpreting compound specific hydrogen isotopic records

We present a systematic study of chain-length distributions and D/H ratios of n-alkyl lipids (both n-alkanes and n-alkanoic acids) in a wide range of terrestrial and aquatic plants around and in Blood Pond, Massachusetts, USA. The primary goal is to establish a model to quantitatively assess the aquatic plant inputs of the mid-chain length n-alkyl lipids to lake sediments and to determine the average hydrogen isotopic ratios of these lipids in different plants. Our results show that middle-chain n-alkyl lipids (C 21–C 23 n-alkanes and C 20–C 24 n-alkanoic acids) are exceptionally abundant in floating and submerged aquatic plants, in contrast to the dominance of long-chain n-alkyl lipids (C 27–C 31 n-alkanes and C 26–C 32 n-alkanoic acids) in other plant types, which are consistent with previously published data from Mountain Kenya and the Tibetan Plateau. Combining available data in different environmental settings allows us to establish statistically robust model distributions of n-alkyl lipids in floating/submerged macrophytes relative to other plant types. Based on the model distributions, we established a multi-source mixing model using a linear algebra approach, in order to quantify the aquatic inputs of mid-chain n-alkyl lipids in lake sediments. The results show that ∼97% of the mid-chain n-alkyl lipids (C 23 n-alkane and C 22 n-acid (behenic acid)) in Blood Pond sediments are derived from floating and submerged macrophytes. In addition, D/H ratios of C 22 n-acid and C 23 n-alkane in the floating and submerged plants from Blood Pond display relatively narrow ranges of variation (−161 ± 16‰ and −183 ± 18‰, respectively). Our study demonstrates that mid-chain n-alkyl lipids such as C 23 n-alkane and C 22 n-acid could be excellent recorders of past lake water isotopic ratios in lakes with abundant floating and submerged macrophyte inputs.

Multi-year landscape-scale assessment of lakewater balances in the Slave River Delta, NWT, using water isotope tracers

We apply a coupled-isotope tracer model to quantify end-of-thaw-season (fall) lakewater balances in the Slave River Delta (SRD), Canada, during 2003–2005, which effectively differentiates the relative importance of hydrological processes across this complex northern freshwater landscape. The model incorporates Great Slave Lake evaporated vapour to the ambient atmospheric vapour pool and is thus tailored to the hydroclimatic setting of the delta, which experiences onshore winds. Results, expressed as evaporation-to-inflow ratios ( E / I ) for 41 delta lakes, reflect the role of spring break-up flooding and local hydrological setting. Fall E / I ratios for lakes where water balances are dominated by exchange with the Slave River or Great Slave Lake are low (0.06–0.53) and do not vary substantially during the three-year monitoring period. E/I ratios for flood-dominated lakes in the active delta are moderate (0.26–0.98) and have low inter-annual variability, even in the absence of spring flooding. This suggests that annual flooding during the spring break-up period is not necessary to maintain positive ( E / I < 1) water balances in flood-dominated lakes, but multiple years without flooding would clearly lead to greater cumulative evaporation. Fall E / I ratios are generally higher and more variable in evaporation-dominated lakes in the relict delta (0.42 to >1), although greater snowmelt runoff tends to occur in sub-sectors with mature spruce forest and offsets open-water vapour loss. Our results indicate that spring inputs (river flooding and snowmelt runoff) are key components of the hydrological evolution of SRD lakes during the open-water season, and distinguish regions of the delta where expected declines in river discharge and climate warming will likely cause lake-level drawdown. Such findings have particular relevance for informed ecosystem management in the Peace-Athabasca-Slave watershed, where unprecedented industrial development is imposing substantial additional pressure on freshwater resources.

129I/ 127I ratios in surface waters of the English Lake District

Accelerator Mass Spectrometry (AMS) was used to measure 129I/ 127I ratios in surface sea, lake, and river water samples collected in 2004 and 2005 from the English Lake District and from SW Scotland, areas which are in relatively close proximity to the Sellafield nuclear fuel reprocessing plant in NW England. The 129I/ 127I ratios in surface water collected from the shore of the Irish Sea were in the range 2.8 × 10 −6 to 8.2 × 10 −6. These ratios are one order of magnitude higher than that of seawater collected from the Irish Sea in 1992, correlating with the increase in 129I content of the Sellafield liquid effluent discharge over the last decade. The 129I/ 127I ratios in lakes in the Lake District were in the range 0.7 × 10 −6 to 6.4 × 10 −6 and decreased exponentially as a function of distance from Sellafield. Consideration of the relative variation of stable I concentrations and 129I/ 127I ratios suggests that Sellafield gaseous discharges may be the dominant source of 129I to the lakes.

Nutrient Enrichment and Planktonic Biomass Ratios in Lakes

Phosphorus (P) to chlorophyll ratios and zooplankton–phytoplankton (Z:P) biomass ratios were assessed in 400 temperate lakes over a gradient of phosphorus (P) and with different fish communities. Most of the lakes in this survey were oligotrophic, with a median total P of 7.3 μg P L−1. Thus, the survey provided information on food web effects during the early phase of eutrophication. There was no tendency toward a reduced yield of autotrophs per unit of P over the gradient covered in this survey. The zooplankton yield per unit of P or chlorophyll a decreased slightly with increased nutrient concentrations, and Z:P biomass ratios decreased with fish community classes, reflecting increased fish predation pressure. However, the variability in biomass ratios within a given range of P and fish class was some 100 times higher than the difference over the gradients. This finding suggests that lake-specific properties, community composition, and food quality are by far the most important determinants of biomass ratios and probably also trophic efficiency in lakes; it further suggests that these factors are superimposed on the general effect of eutrophication, at least up to 30 μg P L−1.

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 μg l−1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 μg C l−1, yet with the majority below 50 μg C l−1and a median of 25 μg C l−1. Total zooplankton biomass displayed a mean and median of 37 and 26 μg C l−1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.